1. Field of the Invention
This invention generally relates to a system and method for reducing holographic noise and, more specifically, to an improved system and method for eliminating noise by controlling the duration of an incident beam during construction of the primary hologram.
2. Description of Related Art
High quality holographic optical elements are used in diffraction optic display systems, such as head up displays (HUDs), for advanced aircraft, helmet mounted displays, laser protective devices, narrow band reflective filters, and holographic high gain screens for simulators. These are only a few of the many uses of high quality reflective holograms.
One of the critical problems in diffraction optic display systems utilizing a replicated hologram has been a degration of the holographic images as a result of the effects of spurious reflection in transmission hologram recordings that are frequency generated during the holographic recording process. Perhaps the most damaging spurious noise holograms are generated by reflections from surfaces which are interfaces of materials of different indexes of refraction, such as air/glass interfaces of the transparent surfaces of a recording cover plate, a hologram substrate, the recording medium itself, and optical elements used to generate the recording beams. These reflections can combine with the primary holographic beams at the recording film to form spurious reflection holograms, when the beams are in the opposite direction, and spurious transmission holograms when the beams are in the same direction. A subsequent display system using this hologram will be degraded by ghost images from the spurious reflection hologram recordings, and rainbow-like flair patterns from the spurious transmission hologram recordings.
The related art has attempted to solve these problems in several different ways. A simple approach has been to form the hologram with energy beams that strike the recording material surfaces at Brewster's angle, at which reflections are at a minimum. This method has very limited application because in general the incident angle is set by the desired holographic function, and is not a variable that one can select at will.
Antireflective coatings on the glass-air interfaces have been used, but the drawbacks include the need to cover a range of input/output angles.
Another approach has been to reduce the spurious reflections by attempting to match the indexes of refraction at the different surface interfaces with an index matching fluid, such as a mineral oil. The entire recording module is immersed in an index of refraction matching fluid bath. The shape of the container is then designed so that surface reflections are directed away from the recording material. For many optical configurations, such a shape does not exist. Furthermore, the phase instability of the optical paths in fluid degrade the desired hologram. Also, the problems of stabilizing the fluid after each plate change, the need to frequently clean the fluid of impurities, and the delays in handling the volume of fluid needed all make this method unsuitable for production.
More sophisticated approaches are disclosed in U.S. Pat. Nos. 4,458,977, 4,458,978, and 4,456,328, in which spurious holograms caused by glass/air interfaces are eliminated by moving an outside cover plate to change the phase of the reflected rays relative to the primary beams during the recording period. Thereby, spurious holograms are not formed. For these approaches that use a moving cover plate, the rate of movement or phase change is a function of the exposure time which depends on the sensitivity of the recording medium. The total movement must provide a phase change of at least one half wavelength in the reflected spurious hologram beams to nullify or "smear out" any spurious interference patterns that would otherwise be recorded. While those systems have advantages, they still have certain disadvantages. For example, the drivers to move the cover plate take time to adjust, calibrate, and test. The complete apparatus is complex to set up, needs long stabilization time, and has many modes of potential failure to decrease yield. Also, since the moving cover plate is another piece of glass and the substrate itself does not have air/glass interface, the substrate cannot have an antireflective coating which is optimized for its final use with an air/glass interface. This necessitates the subsequent addition of a coating or another piece of glass which is coated.
Another attempt to suppress spurious holograms is disclosed in U.S. Pat. No. 3,601,017. Therein, an immersion liquid is applied to either the surface of the recording medium or the surface of a transparent support member, which surface is remote from the direction of light incidence. The thickness of the immersion agent layer is varied over time during an exposure by evaporation or, when it is not so readily evaporated, by generating acoustic or surface waves which, during their reflection, directionally modulate the wave field in the liquid. Preferably, the transparent immersion liquid is O-xylene. This system also has drawbacks, including the fact that the evaporating liquid is not a good optical surface. Further, the variation in thickness would not appear to be highly regulated, particularly when using generated waves.
A need still exists in the technology to provide an improved system and method for producing a high quality hologram that has minimal spurious holograms and at the same time can be produced in an economical and efficient manner.